Process for decolorizing hydrogenated butadiene polymers with a saturated carboxylic acid



PROCESS FOR DECOLORIZING HYDROGENATED BUTADIENE POLYMERS WITH ASATURATED CARBOXYLIC ACID Paul E. Campbell, Bartlesville, Okla, assignorto Phillips Petroleum Company, a corporation of Delaware No Drawing.Application December 27, 1954 Serial No. 477,827

14 Claims. (Cl. 260-851) This invention relates to a process fordecolorizing hydrogenated polybutadiene and hydrogenated butadienestyrene copolymers. In a more specific aspect this invention relates toa process of decolorizing these hydrogenated polymers by treating themwith saturated carboxylic and mineral acids.

In Jones et al. application Serial No. 395,291, filed November 30, 1953,a process of preparing hydrogenated polymers is set forth. Thehydrogenated products are especially valuable because of their lowtemperature characteristics, being flexible at temperatures as low as100 F. and not brittle at temperatures approaching -200 F. Furthermore,they are more resistant to ozone degradation than are the unhydrogenatedmaterials and have other improved physical properties. They arethermoplastic and can be vulcanized, the products being used to makefibers, films, dishes, coating materials, protective coverings, etc. Forthe hydrogenation, the polymers, substantially free of salts or othermaterials which act as hydrogenation catalyst poisons, are charged to ahydrogenation reactor in the form of a solution or dispersion in asuitable solvent.

Suitable solvents include saturated cyclic hydrocarbons such ascyclohexane, methylcyclohexane, decalin, and the like. Aromatichydrocarbons, such as benzene and toluene, cyclic ethers, such asdioxane; and paraffinic hydrocarbons, such as isooctanes, isoheptane andnormal heptane can also be used. Mixtures of solvents can be used ifdesired.

After the polymer has been dissolved in a solvent, a nickel catalyst isadded, hydrogen is introduced and the temperature is raised to asuitable hydrogenation temperature, the hydrogenation being carried outin a batch- Wise or a continuous process. An efiicient way of removingthe hydrogenation catalyst is by passing the solution through a magneticseparator to remove the magnetizable catalyst. Processes employing otherfilters or centrifuges have also been used.

The polymers used as starting materials are selected from homopolymersof butadiene and copolymers of butadiene and styrene, prepared frommonomer mixtures comprising not over 30 parts by weight of styrene per100 parts by Weight of monomers. The polymers are prepared by emulsionpolymerization at temperatures between F. and 140 F., preferably 20 to60 F. In this emulsion polymerization, it has been found thatapproximately 75 to 85 percent of the butadiene polymer is formed as aresult of l-4 addition while 15 to 25 percent of the polymer is formedas a result of l-2 addition. The Mooney viscosity of the polymer ispreferably below 40 (ML-4) at 212 F.

Best results are obtained using a finely divided nickelkieselguhrcatalyst having a particle size of from 1 to 8 microns which has beenactivated at a temperature between 500 and 800 F. for a period ofseveral hours by passing hydrogen thereover. Activation of such acatalyst at 600 F. for four hours using approximately 100 volumes ofhydrogen per volume of catalyst, has been found atent to be particularlysuitable. A greater amount of hydro gen, up to 1000 volumes of hydrogenper volume of catalyst, is sometimes used. Treatment with hydrogenreduces at least a part of the nickel to elemental nickel, generally 35to 40 percent of the nickel being reduced. The reduced nickel content ispreferably in the broad range of from 10 to 50 percent reduced nickelbut some catalysts have a reduced nickel content of as high as percent.Since the catalyst is charged on an unreduced basis, the weight ofreduced catalyst is calculated and multiplied by the reduced nickelcontent to obtain the weight of the reduced nickel set forth above. Forhydrogenation of the polymer, 2 to 30 Weight percent on the unreducedbasis of catalyst based upon the weight of polymers gives the preferredrate of hydrogenation.

Reaction pressures for the hydrogenation range from atmospheric to 3000p.s.i.g., the usual range being in to 1000 p.s.i.g. The temperature canrange from 75 F. up to the degradation temperature of the polymer,maximum temperatures ranging as high as 700 to 1000 F. The preferredrange is between 300 and 600 F. Reaction times in the range of l to 25hours, preferably 2 to 8 hours can be employed.

To obtain a significant improvement in the properties of the polymer,the unsaturation should be reduced to a value of at least 50 percent ofthe original unsaturation and completely hydrogenated products can beobtained. However, when the polymer is completely hydrogenated it is nolonger vulcanizable. Unsaturation in the range of 10 to 30 percent givesproducts with the best balance of physical properties.

This process and examples of materials produced thereby are more fullyset forth in the above-identified I ones et al. application.

One dilficulty which has been observed in the production of thesehydrogenated polymers is that the products are sometimes gray or blackin color. In molded samples, the gray or black color sometimes appearsas opaque streaks or spots.

I have found a method by which these oil-color products can be treatedin order to remove the dark color and to provide materials which areclear or which have no more than a trace of yellow therein. The processof my invention comprises treating the polymer with an organic monoorpolycarboxylic acid containing 2 to 8 carbon atoms or with a mineralacid. The products can be treated as a solution or as a sheet or film ofthe material. An increase in light transmission, as much as 36 percent,has been obtained by this method, light transmission determinationshaving been made on molded discs of polymer using a Lumetron Colorimetersold by the Schaar Company of Chicago.

Therefore, the following are objects of this invention:

An object of this invention is to provide a method for decolorizinghydrogenated polymers of butadiene. A further object of this inventionis to provide a method for decolorizing hydrogenated polybutadiene orhydrogenated butadiene-styrene copolymers by treatment with saturatedmonoor polycarboxylic acids containing 2 to 8 carbon atoms or mineralacids.

Other objects and advantages of this invention Will be evident to oneskilled in the art upon reading this disclosure.

As stated, the polymer is decolorized by treatment with a suitable acid.The acids include the carboxylic acids containing 2 to 8 carbon atomssuch as acetic, propionic, butyric, valeric, caproic, heptanoic, andcaprylic acids; dicarboxylic acids such as oxalic, malonic, succinic,glutaric, adipic, pimelic, azelaic, and sebacic acids. Mineral acidssuch as hydrochloric, phosphoric, and the like are also suitablealthough they must be used in concentrations below that at whichreaction with the polymer occurs. Attempts to use formic acid have notbeen satisfactory in color removal.

The polymer can be treated with the acid while it is dispersed ordissolved in a solvent or the polymer can be treated while it is in asolid form of considerable size.

The acid is used in an amount from 0.01 to 5 percent of the weight ofthe polymer in most cases. Of course, larger amounts can be used, theupper limit being dictated by economy. Less time is usually required fordecolorization when larger amounts of the acid are used.

While the polymer can be decolorized at room temperature and attemperatures as low as 35 F., the action is relatively slow and it ispreferred to operate at temperatures within the range of 150 to 185 F.Temperatures up to the boiling point of the liquids present aresometimes used but are seldom necessary.

When the polymer is decolorized in the form of a solution, the solutionsare usually quite dilute in order to eliminate the necessity of workingwith viscous solutions. I have found it desirable to operate withapproximately 3 to weight percent solutions. The same solvents used forthe hydrogenation can be used for the decolorization and, in fact, oneconvenient method of carrying out the invention is to decolorize thesolution from the hydrogenation operation directly after removing thecatalyst.

The organic acids which are used to effect the decolorization of thehydrogenated polymer can be used in a substantially pure form or can bediluted with water and/ or an organic solvent. In some cases thepresence of a small amount of water increases the decolorizing activityof the acid.

The mechanism by which the decolorization takes place is not known.There is no visible indication of a reaction taking place. It issurprising therefore that such a small amount of acid is effective.Analysis of the polymer before and after decolorization shows the samenickel content and, thus, the reaction is not one of removal of themetal. Likewise, the result is not a function of the hydrogen ionconcentration, because the addition of a base does not cause the colorto reappear.

The treatment does not change the physical properties of the polymer.Measurements of tensile strength, elongation, softening point, and flextemperature show l ttle, if any, change. Inherent viscosities are alsosubstantially unchanged.

The following are specific examples of the process of my invention whichillustrate preferred methods of operation. They should be considered asillustrative only.

EXAMPLE I The light transmission of a discolored molded disc ofhydrogenated polybutadiene of 8.9 percent unsaturation, prepared byhydrogenating a methylcyclohexane solution of a 25 (ML-4) Mooneypolybutadiene with a nickelkieselguhr catalyst, was measured. This discwas employed as a control in this example. Several solutions of the samehydrogenated polybutadiene in methylcyclohexane were then prepared using25 grams of the hydrogenated polymer per 600 cc. of methylcyclohexane.To each of these solutions were added, with stirring, 100 cc. ofraceticacid in the concentrations shown in Table 1. After the desired length oftime, isopropyl alcoholwas added to precipitate the hydrogenatedpolybutadiene. The precipitate was then washed several times withalcohol and vacuum dried at 50 F. Discs molded from the treatedhydrogenated polybutadiene were'then measured for light transmissionusing the Lumetron Colorimeter to determine the difference in lighttransmission between the treated and the untreated material. Results aretabulated below: i H

4 Table 1 LIGHT TRANSMISSION OF HYDROGENATED POLYBUTA- DIENE TREATEDWITH ACID cc.Acetic Concentra. Increasein Color of Disc Acid pertionofAque- Treating Light 25 Grams ous Acetic Time, Trans- Hydrogen-Acid, Vol- Hours mission, Unated Polyume percent Percent treated Treatedbutadiene 100(glacial) 1 33-36 Gray Colorless. 75 1 3336 do Do. 2-333-36 do. Do. 24-27 3336 do Do. 30-33 33-36 do. Do.

Except for the dilference in color, the discs appeared to be similar tothose prepared from untreated material.

EXAMPLE II Solutions in methylcyclohexane of a hydrogenatedpolybutadiene of 31.8 percent unsaturation prepared from a 41 F. 20Mooney polybutadiene with a nickel-kieselguhr catalyst were decolorizedby treatment with an excess of glacial acetic acid as in Example I. Thesolid product, obtained by drum drying of the solution, was molded intosuitable forms for measuring various physical prop erties. Theproperties were compared with those of an untreated sample. Results aregiven in Table 2.

1 The softening point was determined by measurements on a simplifiedGoodrich Plastometer (Ind. Eng. Chem., Anal. Ed. 2, 96 (1931)). Theplasticity was plotted against temperature and the softening pointdetermined by extrapolation.

I The temperature where the apparent modulus of elasticitry=135,000p.s.i. as measured on a Tinius Olsen Stiflness tester according to ASTM:D1043-49T.

Treatment of a sample of hydrogenated polybutadiene with acetic acidcaused little change in inherent viscosity. The inherent viscosity ofthe untreated sample was 0.887

and of the treated sample 0.835.

EXAMPLE III Gray-colored molded discs of hydrogenated polybutadiene0.075 inch in thickness were immersed for about 72 hours at roomtemperature in about 10 parts by weight of each of the following acids:acetic, propionic, butyric, valeric, caproic, heptanoic, and caprylicacids. At the end of this period of time, the discs had in each casebecome transparent, and had lost their gray color. The acids used weretechnical grade materials, substantially free of water.

EXAMPLE IV A series of runs was made in which gray-colored solutions ofhydrogenated polybutadiene in methylcyclohexane were treated withdifferent acids. In each case a solution containing 5 percent by weightof the hydrogenated polybutadiene was heated to F., 1 part by weight ofacid per one part of polymer was added, and the container shaken. Whendiluted acid was used, the amount added was based on acid prior todilution. The acids tested were concentrated hydrochloric acid,hydrochloric acid containing 33 /2 percent water, glacial acetic acid,and acetic acid containing 33% percent water. The two acetic acidtreated portions were colorless following treatment. The twohydrochloric acid treated portions were slightly yellow.

EXAMPLE v Dark-colored methylcyclohexane solutions of hydrogenatedpolybutadiene were treated with acetic acid at different temperatures inorder to determine the efiect of temperature on the decolorizationtreatment. The tests were made at 180 F. and at room temperature.

In the first run, 426 grams of hydrogenated polybutadiene was added tosufiicient methylcyclohexane to make 15 liters of solution. Ten litersof this solution were then heated to 180 F. and to it was added 12.1 ml.of glacial acetic acid and 57.5 ml. of water. The solution was stirredfor 30 minutes. At the end of that time, the solution appearedcolorless.

In the second run, a solution was similarly prepared. To liters of thissolution maintained at room temperature it was necessary to increase theamount of acetic acid added to 150 ml. in order to effectdecolorizatio-n in one hour.

In an additional run, to ten liters of a solution prepared as before andheated to 180 F., was added 1 ml. of glacial acetic acid. The solutionwas stirred for 10 minutes. At the end of this time the solutionappeared completely colorless.

EXAMPLE VI A 41 F. polybutadiene rubber having a Mooney value of 20(ML-4) was hydrogenated at 400 F. using a nickel-kieselguhr catalyst.The product had an unsaturation of 1.0 percent, a tensile strength of1990 p.s.i.g. and an elongation of 725 percent. A portion of thismaterial was molded into a inch thick sheet. This sheet was then cut intwo and one-half of it was immersed in glacial acetic acid for 72 hoursat 78 F., washed with water, and dried under vacuum at 60 C.

A portion of this molded sheet, before and after treatment, was analyzedfor nickel content and it was found that the nickel content was the samewithin the limit of experimental variation.

It will be evident to those skilled in the art that variousmodifications of this invention can be made, or followed, in the lightof the foregoing disclosure without departing from the scope of myinvention.

I claim:

1. A method of decolorizing a polymer prepared by emulsionpolymerization selected from group consisting of homopolymers ofbutadiene and copolymers of butadiene and styrene, prepared from monomermixtures containing not over thirty parts by weight of styrene per 100parts by weight of monomers, said polymers having a gray color as aresult of the reduction of the residual unsaturation by at least 50percent by hydrogenating same in the presence of a nickel catalyst, saidmethod comprising contacting the polymer with an acid selected from thegroup consisting of saturated unsubstituted carboxylic acids containing2 to 8 carbon atoms.

2. The method of claim 1 wherein said polymer is a copolymer ofbutadiene and styrene prepared from a. monomer mixture containing notover thirty parts by 6 weight of styrene per one hundred parts by weightof monomers.

3. The method of decolorizing a homopolymer of butadiene prepared :byemulsion polymerization, said polymer having a gray color as a result ofthe reduction of the residual unsaturation by at least 50 percent byhydrogenating same in the presence of a nickel catalyst, said methodcomprising contacting a polymer with an acid selected from the groupconsisting of saturated unsubstituted carboxylic acids containing 2 to 8carbon atoms.

4. The process of claim 3 wherein said acid is acetic acid.

5. The process of claim 4 wherein an aqueous solution of said aceticacid is used.

6. The process of claim 3 wherein said acid is propionic acid.

7. The process of claim 3 wherein said acid is butyric acid.

8. The process of claim 3 wherein said acid is valeric acid.

9. The process of claim 3 wherein said acid is caproic acid.

10. The method of decolorizing a homopolymer of butadiene prepared byemulsion polymerization, said polymer having a gray color as a result ofthe reduction of the residual unsaturation by at least 50 percent byhydrogenating same in the presence of a nickel catalyst, said methodcomprising dissolving said polymer in a solvent and contacting theresulting solution with an acid selected from the group consisting ofsaturated unsubstituted carboxylic acids containing 2 to 8 carbon atoms,amount of said acid being 0.01 to 5 percent by weight based upon theweight of said polymer.

11. The method of decolorizing a homopolymer of butadiene prepared byemulsion polymerization, said polymer having a gray color as a result ofthe reduction of the residual unsaturation by at least 50 percent byhydrogenating same in the presence of a nickel catalyst, said methodcomprising contacting said polymer in solid form with an acid selectedfrom the group consisting of saturated unsubstituted carboxylic acidscontaining 2 to 8 carbon atoms.

12. The process of claim 11 wherein said acid is acetic acid.

13. The process of claim 3 in which said acid is heptanoic acid.

4. The process of claim 3 in which said acid is caprylic ReferencesCited in the file of this patent UNITED STATES PATENTS 2,115,564 Thomaset a1. Apr. 26, 1938 2,160,931 Wiley June 6, 1939 2,693,461 Jones Nov.2, 1954- 2,751,277 Coover et a1 June 19, 1956 2,813,809 Jones et a1 Nov.19, 1957

1. A METHOD OF DECOLORIZING A POLYMER PREPARED BY EMULSIONPOLYMERIZATION SELECTED GROUP CONSISTING OF HOMOPOLYMERS OF BUTADIENEAND COPOLYMERS OF BUTADIENE AND STYRENE, PREPARED FROM MONOMER MIXTURESCONTAINING NOT OVER THIRTY PARTS BY WEIGHT OF STYRENE PER 100 PARTS BYWEIGHT OF MONOMERS, SAID POLYMERS HAVING A GRAY COLOR AS A RESULT OF THEREDUCTION OF THE RESIDUAL UNSATURATION BY AT LEAST 50 PERCENT BYHYDROGENATING SAME IN THE PRESENCE OF A NICKEL CATALYST, SAID METHODCOMPRISING CONTRACTING THE POLYMER WITH AN ACID SELECTED FROM THE GROUPCONSISTING OF SATURATED UNSUBSTITUTED CARBOXYLIC ACIDS CONTAINING 2 TO 8CARBON ATOMS.